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NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 7
of special importance is the retractor muscles of the foot and byssus.
These muscles, which carry the visceral mass and can contract it against
the shell and move the foot, should certainly be the most important ones
for bivalves as well as for univalves. Whatever the origin of the pelecy-
pods may have been, as a larval partial duplication from a gastropod
origin or directly from the hypothetical ancestors of the mollusks,
retractor muscles must have been the primary muscles of the animal. The
adductors, usually the strongest muscles of an ordinary bivalve, seem to
be enlarged parts of the muscles fastening the mantle to the shell. The
position of the retractor of the foot compared to the posterior retractor
of the byssus, and the form and placement of the anterior retractor in
regard to the umbo, give valuable clues for classification (cfr. text-fig.
15).
The shell surface of most of the mytilid species is divided more or
less distinctly into three areas, either by different sculpture or coloring.
The anterior part, the lunule, usually is radiately ribbed, grooved, or
striated, the grooves often forming teeth on the anterior margin; or the
lunule is turned inwards so that the ridges form tooth-like thickened parts
with corresponding grooves in the opposite valve. Sometimes a lunule is
seen as a duller part of the periostracum (cfr. text-fig. 47), but often
it is not separable from the rest of the shell. The median part generally
is smooth and furnished with a more shining periostracum than the rest
of the shell. The posterior part, usually bordered by a more or less distinct keel from the umbones to the posteroventral angle, is more heavily
sculptured, has a different color, or hairy periostracum, or differs otherwise from the rest of the shell. The margins may be smooth or crenulated
more or less extensively, generally in accordance with the sculpture.
Several species have margins partly crenulated when young but smooth
when full grown. The ligament is deejj|§et and a real escutcheon is not
developed. The ligament is supported by a sometimes solid nymphae and
the lower or ventral fibrous part, the resilial part, furnished with calcareous needles, is fastened to a ridge, here named the resilial ridge,
which is white and of a quite different consistency than the rest of the
valves (cfr. Pl. 5, fig. 23). This ridge continues posteriorly to the dorsal
margin and forms one of the best characteristics of the family Mytilidae.
The mantle is usually separated for the entire length of the shell,
from the anterior adductor to the middle of the posterior margin. An
excurrent opening, a slit in the septum connecting the two mantle lobes
or a more or less distinct tubuliform siphon, is separated from the free
mantle lobe by a short contiguous part and a septum extending ventral-

NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 7
of special importance is the retractor muscles of the foot and byssus.
These muscles, which carry the visceral mass and can contract it against
the shell and move the foot, should certainly be the most important ones
for bivalves as well as for univalves. Whatever the origin of the pelecy-
pods may have been, as a larval partial duplication from a gastropod
origin or directly from the hypothetical ancestors of the mollusks,
retractor muscles must have been the primary muscles of the animal. The
adductors, usually the strongest muscles of an ordinary bivalve, seem to
be enlarged parts of the muscles fastening the mantle to the shell. The
position of the retractor of the foot compared to the posterior retractor
of the byssus, and the form and placement of the anterior retractor in
regard to the umbo, give valuable clues for classification (cfr. text-fig.
15).
The shell surface of most of the mytilid species is divided more or
less distinctly into three areas, either by different sculpture or coloring.
The anterior part, the lunule, usually is radiately ribbed, grooved, or
striated, the grooves often forming teeth on the anterior margin; or the
lunule is turned inwards so that the ridges form tooth-like thickened parts
with corresponding grooves in the opposite valve. Sometimes a lunule is
seen as a duller part of the periostracum (cfr. text-fig. 47), but often
it is not separable from the rest of the shell. The median part generally
is smooth and furnished with a more shining periostracum than the rest
of the shell. The posterior part, usually bordered by a more or less distinct keel from the umbones to the posteroventral angle, is more heavily
sculptured, has a different color, or hairy periostracum, or differs otherwise from the rest of the shell. The margins may be smooth or crenulated
more or less extensively, generally in accordance with the sculpture.
Several species have margins partly crenulated when young but smooth
when full grown. The ligament is deejj|§et and a real escutcheon is not
developed. The ligament is supported by a sometimes solid nymphae and
the lower or ventral fibrous part, the resilial part, furnished with calcareous needles, is fastened to a ridge, here named the resilial ridge,
which is white and of a quite different consistency than the rest of the
valves (cfr. Pl. 5, fig. 23). This ridge continues posteriorly to the dorsal
margin and forms one of the best characteristics of the family Mytilidae.
The mantle is usually separated for the entire length of the shell,
from the anterior adductor to the middle of the posterior margin. An
excurrent opening, a slit in the septum connecting the two mantle lobes
or a more or less distinct tubuliform siphon, is separated from the free
mantle lobe by a short contiguous part and a septum extending ventral-